The present invention relates to a process for the continuous production of isocyanate polyaddition products containing urethane groups. These polyaddition products are made from branched or cross-linked distillation residues which residues are substantially free from monomer but still contain free isocyanate groups and short-chain compounds containing alcoholic hydroxyl groups. Such distillation residues may be obtained from the commercial production of tolylene diisocyanates. The polyaddition products obtained in accordance with the present invention are useful as starting materials for the production of plastics (e.g., molding compositions) and as reactive fillers for a variety of plastics because they impart increased flame resistance and improved physical properties to the product plastics.
The distillation of the phosgenation product of tolylene diamines under industrial conditions is accompanied by formation of relatively high molecular weight, insoluble derivative products containing uretdione, isocyanurate, carbodiimide, uretone imine, urea and biuret groups. Methyl benzimidazolones formed during phosgenation are also gradually biuretized with the free isocyanate groups present to form insoluble, cross-linked products.
In recent years, these distillation residues from the industrial production of tolylene diisocyanate (hereinafter referred to as "TDI") have been partly recycled by alkaline hydrolysis. However, only a relatively small proportion of tolylene diamine is recovered by this extremely slow hydrolysis reaction after an expensive purification process.
The long-tube vertical evaporator described in DE-OS No. 2,035,731 has been more successful in maximizing the yield of 2,4-TDI ("T 100") and of isomer mixtures of 80% of 2,4-TDI and 20% of 2,6-TDI ("T 80") and of 65% of 2,4-TDI and 35% of 2,6-TDI ("T 65"), based in each case on the tolylene diamine used. Such evaporators produce reduced quantities of a sump phase which is substantially free from monomers yet still contains approximately 25% by weight of free isocyanate groups (hereinafter referred to as "TDI residue tar"). This sump phase is frequently "denatured" with water so that the free isocyanate groups still present in the TDI residue tar react with the water to form carbon dioxide and additional urea and biuret groups. After this denaturing, only a small amount (generally about 1 to 10% by weight) of free isocyanate groups is left over. These cross-linked, substantially monomer-free products treated with water are hereinafter referred to as "denatured" TDI distillation residues. On storage in water or in moist form, the isocyanate content of the denatured TDI residue gradually decreases over long periods of time.
The slag-like denatured TDI residues thus obtained, contain polyurethane, biuret, uretdione, carbodiimide and isocyanurate groups. These denatured residues are completely insoluble in all standard solvents unlike the comparatively low molecular weight (but non-storable) sump phase rich in NCO-groups before denaturing with water. Such denatured TDI residues begin to melt at temperatures above 250.degree..+-.30.degree. C., and decompose with evolution of gas. These denatured TDI residue tars may be subjected to subsequent chemical modification and mechanical size reduction to convert them into starting materials and reactive fillers for the production of plastics.
TDI distillation residues have also been subjected to polyaddition reaction between the TDI residue tar and a low molecular weight polyol. If this polyaddition reaction is carried out by the known casting process, an almost explosive exothermic reaction takes place when ethylene glycol or diethylene glycol is added because this operation is carried out at temperatures above the melting point of the TDI residue tar (generally above 100.degree. C.). Even when less than 0.5 kg of residue are reacted, the temperature rises above 200.degree. C. and the residue partially decomposes and becomes brittle. Use of greater amounts of residue in the reaction mixture may result in temperatures high enough to cause smoldering of the decomposition products.
Use of an inert solvent to dissipate the heat of reaction has been found to be impractical because the cross-linked polyaddition products formed during the reaction accumulate a coarse slag from which the liquid reaction medium cannot be removed to the point of odorlessness, even after expensive size reduction. Additionally, such solvents are ecologically and economically disadvantageous.